The present invention relates to dimensionally-stable polymer films. The present invention also relates to magnetic media using the films.
PET (polyethylene terephthalate) and PEN (polyethylene naphthalate) have been used as base films for magnetic recording media. Use of aromatic polyamide films having higher thermal resistance and higher rigidity in base films for magnetic recording media is also proposed. In recent years, in videotapes and magnetic recording media for recording data, trends toward higher recording capacity have been in rapid development, and the recording density and the recording area tend to increase to achieve higher recording capacity. That is, the recording wavelength has shifted to a shorter wavelength side and the track width has decreased to achieve high recording density. As a result, a change in tension during recording/reproducing and a slight change in the size of a film during a starting mode or a stopping mode cause a problem in that normal recording/reproducing is not achieved. On the other hand, due to the increase of recording/reproducing speeds on/from magnetic recording media, the tension applied to magnetic tapes has also become larger than before. The thickness of a magnetic recording medium tends to decrease to increase recording area, and thus, a base film thereof must have a significantly small thickness and high dimensional stability, which have not been achieved.
Conventional primary means for improving dimensional stability is an increase in strength of the film. For example, in thermoplastic films, such as polyester films, the strength has been improved by increasing the stretching ratio. Since the improvement in the stretching ratio approaches its limit, aromatic polyamide films and particularly para-oriented aromatic polyamide films have been studied. For example, Japanese Unexamined Patent Publication No. 62-70421 discloses an aromatic polyamide film having a tensile modulus of 1,800 kg/mm2 or more in at least one direction and a coefficient of hygroscopic expansion of 8xc3x9710xe2x88x926 mm/mm/RH % or less, and a method for performing uniaxial stretching at a high stretching ratio at a high temperature in order to obtain the film. Although this method is substantially effective for improving dimensional stability in the stretching direction, the modulus and the dimensional stability in a direction perpendicular thereto are decreased in some cases, and a high level of compatibility of the dimensional stability between the longitudinal direction and the transverse direction is not achieved. Moreover, a method for ensuring dimensional stability in the transverse direction when a force is applied in the longitudinal direction has not been suggested or described. Although such an example is disclosed in Japanese Unexamined Patent Publication No. 62-70450 this method also includes the same problems.
Japanese Unexamined Patent Publication No. 59-45124 discloses an aromatic polyamide film in which the product of the heat shrinkage and the coefficient of thermal expansion is 1.0xc3x9710xe2x88x927 to 1.0xc3x9710xe2x88x924 (%/mm/mm/xc2x0 C.), and Japanese Unexamined Patent Publication No. 10-222837 discloses an aromatic polyamide film having a strength of 20 to 60 kg/mm2, a tensile modulus of 1,000 to 2,500 kg/mm2, a heat shrinkage rate of 2% or less, and a dimensional stability at 50xc2x0 C. and 80% RH % of 0.1% or less. However, these publications do not suggest nor disclose ensuring dimensional stability when a force is applied in the longitudinal direction.
Examples of the controlling the ratio of tensile moduli of the longitudinal direction to the transverse direction are Japanese Unexamined Patent Publication Nos. 56-11624 and 62-62424. Japanese Unexamined Patent Publication No. 56-11624 discloses a magnetic recording medium comprising an aromatic polyamide film, provided with a magnetic recording layer, having a tensile modulus in the transverse direction which is at least 1.3 times the tensile modulus of the longitudinal direction, for the purpose of preventing bending and wrinkling of VTR tapes. Japanese Unexamined Patent Publication No. 62-62424 discloses a magnetic recording medium comprising an aromatic polyamide film, provided with a magnetic recording layer, having a tensile modulus in the longitudinal direction which is at least 1.3 times the tensile modulus of the transverse direction, for the purpose of preventing jitter (fluctuation of images due to shrinkage/elongation of the tape) during the VTR reading mode. These examples intend improvement in dimensional stability in the transverse or longitudinal direction, but does not always ensure the dimensional stability in the direction other than the strengthened direction when an external force is applied.
That is, when a force is applied to a film, infinitesimal deformation of the film will occur not only in the direction of the force but also in a direction perpendicular to the direction of the force. In order to ensure superior write/read characteristics as a high-density magnetic recording medium, deformation not only in the direction of the force but also in a direction perpendicular to the direction of the force must be strictly controlled. Balance between physical parameters, such as tensile modulus, heat shrinkage rate, and coefficient of hygroscopic expansion, which has been conventionally performed, has already reached its limits.
The present inventors have carefully observed a change in size of a film when a force is applied and have discovered that such a behavior significantly contributes to the performance of the magnetic recording medium, and that a change in size in the direction which is perpendicular to the direction to which a force is applied can be controlled by fundamentally reinvestigating film manufacturing conditions which have been considered to be directly and exclusively determined depending on the polymer (particularly in the aromatic polyamide).
It is an object of the present invention, for solving the above problems based on such knowledge, to provide a polymer film (specifically an aromatic polyamide film) having high dimensional stability and a magnetic recording medium having electromagnetic conversion characteristics which cause always no reading errors in a reproducing mode.
The present invention relates to a polymer film characterized by a Poisson""s ratio of the film of the transverse direction (TD) to the longitudinal direction (MD) of less than 0.4.